Because of the greatly increased and still growing need for single-crystal silicon as a starting material of solar cells and for electronics, methods are being developed with which single crystals of large dimensions, having the fewest possible impurities, can be produced cost-effectively. Thus, various methods for producing single crystals of this type are conventional and have been described.
A conventional and widespread standard method for producing single crystals is the Czochralski method, in which the semiconductor material which is to be pulled to form a single crystal is melted in a quartz crucible, and the crystal is pulled upwards from this melt. The major drawbacks of this method are that the melt always attacks the crucible, contaminating the melt and thus the crystal, and the quartz crucible has to be replaced after a relatively short operating life. There are also solutions for achieving a longer operating life for the quartz crucible by complex methods, as is described in DE 102 17 946 A1. However, only a crystal having the impurities typical of CZ is ever pulled, and said impurities are also distributed axially inhomogeneously.
DE 24 16 489 describes a method for growing single crystals composed of silicon, which are low in or free of inclusions, by the Czochralski method, in which the drawbacks of the Czochralski method are supposed to be overcome by using a melted supply rod as a material source instead of a crucible, analogously to the pedestal method. When an inductor is used as a heat source and the frequency of the alternating current flowing through the inductor is selected in an adapted manner, a solid edge layer is not melted, as a result of the current displacement in combination with the lateral heat losses on the surface of the supply rod, and thus forms a vessel for the melt and thus makes it possible to pull a crystal free from foreign crucible materials, similarly to the pedestal method. A major drawback of this solution is that only crystals having a substantially smaller diameter than the supply rod can be pulled. Moreover, in this solution, based on the relatively good conductivity of the compact material, the non-melting edge of the supply rod has to give off a large amount of heat in order to be stabilised, and this heat has to be compensated by the inductive energy supply.
The method described in DE 44 47 398 is also supposed to overcome the drawbacks of the Czochralski method. For this purpose, the semiconductor material located in the crucible is initially melted using an induction coil. This initially results in a melt, which fills the crucible. The heating power is subsequently reduced until a stable solid layer forms on the inner crucible wall. The solidified edge layer acts as a passivation layer, and as a base body additionally provides automatic stabilisation of the melt temperature. During the phase of super heating the melt, it enters a reaction with the outer wall of the crucible, causing oxygen and other impurities to enter the melt. As in the previous example, in this solution too a large amount of energy has to be emitted to the cooler environment in the edge regions, so as to stabilise the solid edge layer in this case too. The volume of the crystal to be produced with the proposed apparatus is limited, since the lack of options for super heating the melt means that recharging with solid Si is not possible, and recharging with liquid silicon would lead to the inclusion of foreign substances.
A further method is the floating zone (FZ) method, with which high-purity single Si crystals can be pulled. DE 30 07 377 A1 describes the basic principles of the FZ method. A drawback in this case is the limited diameter and the high production costs of adapted polycrystalline raw rods.
A third method for producing single crystals is the pedestal method, which also does not use a crucible. A polycrystalline rod is melted by an inductor over the entire upper end face thereof. The melted zone is brought into contact with a seed crystal through the opening of the inductor and, unlike in the zone melting method, the growing single crystal is pulled upwards. A major drawback of the pedestal method is that the diameter of the pulled crystal is always much smaller than that of the inductor opening and that of the raw rod.
The main method steps and apparatus features which are characteristic of the pedestal method are described in DE 21 10 882 A1. A polycrystalline rod consisting of semiconductor material is melted at the crest thereof by a heater, from which a single crystal is subsequently pulled.
A further example of pulling a single crystal by the pedestal method is described in detail in CA 713524 A.
U.S. Pat. No. 2,961,305 A describes a solution for pulling a single crystal by the pedestal method.
The major drawback of the conventional pedestal method is that the pulled single crystal always has a smaller diameter than the raw rod used. The raw rod in turn has a limited diameter, determined by the Siemens method for producing silicon. In particular, for a large crystal diameter, the ratio of said crystal diameter to the supply rod diameter becomes even more unfavourable, for heating-related reasons. Further solutions include using the pedestal method for overcoming these drawbacks.
Thus, DE 35 19 632 A1 describes a method and an apparatus for pulling monocrystalline silicon rods in which a silicon granulate is filled into a vessel, in which a melt lake, from which the single crystal can be pulled, is to be produced on the surface. To produce the melt lake, the granulate is heated, using the flow of current between two silicon electrodes penetrating into the granulate, until a melt lake is formed. However, this method cannot be carried out in practice, since the electrodes, which have to be made of silicon so as not to contaminate the resulting melt and thus the crystal to be pulled, have a specific electrical resistivity about 30 times that of the resulting melt at the melting point. Thus, per unit volume, only approximately 3% of the supplied electrical energy is converted to heat in the melt lake, whilst the remaining approximately 97% of the electrical energy has to be released in the solid silicon electrodes, and this means that either said electrodes would melt themselves or the melt would solidify.
A further example is described in DE 35 25 177 A1. In this case, the silicon electrodes are inserted into holes, which are formed in a solid silicon block. By heating the silicon electrodes and cooling the silicon block, an arc is to be produced between two silicon electrodes and be ignited by a third silicon electrode. The drawback of this solution is in particular that the electrodes additionally have to be heated and the silicon block has to be cooled. It is also doubtful whether the two electrodes in the proposed arrangement would actually, in practice, form a melt lake from which a silicon single crystal can subsequently be pulled. The energy problem described above in relation to DE 35 19 632 A1 applies in this case too.
DE 43 18 184 A1 describes a method and an apparatus for pulling single crystals from a melt. The method provides that a monocrystalline seed crystal evolves into a single crystal in that the seed crystal is dipped into the melt and raised vertically from the melt in a controlled manner. The melt forms a melt lake, which is held on a solid support body, produced from the semiconductor material, merely by the surface tension and by electromagnetic forces in an induction coil. As the single crystal grows, semiconductor material in solid or liquid form is recharged into the melt. Additionally heating the support body effectively assists the induction coil, which acts as the heat source for producing and/or maintaining the melt lake, and heat losses which necessarily occur when the semiconductor material is recharged can thus be compensated. The induction coil is preferably planar and is located above the support body, at a variable distance therefrom. The melt lake rests on the support body, which can be heated by a resistance heater which is arranged in a cavity of the support body as an additional heat source. The support body is preferably assembled from a plurality of segments rather than being formed from a single piece. Adapted shaping of the individual segments provides a cavity, for receiving the additional resistance heater, in the support body. The drawback of this solution is in particular that the support body has to be manufactured from silicon in a highly complex manner, and has to be returned to the original state thereof by further mechanical machining after the pulling process is complete. Moreover, the resistance heater leads to a considerable additional energy requirement. Since the support body is manufactured from solid silicon, additional measures are required so as to maintain a temperature gradient between the resistance heater and the melt lake, in such a way that the melt volume and the support body itself simultaneously remain stable.